1. Field of the Invention
This invention relates to an apparatus for fixing workpieces to be machined, comprising at least two individually adjustable jaws, which are preferably adapted to be fixed in position, and to method of fixing a workpiece by means of such fixing apparatus.
2. Description of the Prior Art
Fixing apparatuses, such as back rests or rotary drives in machine tools, are known in various forms and may comprise jaws which are movable in unison so that the workpiece can be fixed in a centered position, or may comprise individually adjustable jaws so that the workpiece can be fixed by a flat- or sag-compensating clamping sequence whereas eccentricities are maintained. It has not been possible before to use one and the same fixing apparatus for fixing a workpiece in a centered position and for fixing a workpiece in a flat- or sagcompensating clamping sequence and it is not possible to monitor the fixing operation so as to detect a displacement of the workpiece under the clamping pressure, the accuracy with which the clamping jaws are adjusted, the movement of the jaws into engagement with the workpiece and the yielding movement performed under the clamping pressure. From German Patent No. 30 31 029 C3 it is known to provide one of the jaws of a back rest with an indicating device for detecting the location of the point of contact between the jaw and the workpiece and to provide also a displacement sensor for visually indicating the distance over which the jaw has been displaced. But in that case it is merely possible to move the jaw over a calculated or empirically determined distance from the indicated location of the point of contact in order to compensate a sag of the workpiece. But where a high accuracy is required, the result which can thus be achieved is not satisfactory owing to the yielding movement which is due to internal consolidation of the fixing device, the fact that the location of the point of contact is indefinite owing to local surface irregularities and the fact that the friction conditions cannot be taken into account in the adjustment of the jaws. In an endeavor to take the friction conditions and the yielding movements performed by the jaws under the clamping pressure into account it has already been proposed that in the operation by which the jaws are moved into engagement with the workpiece one of the jaws should be advanced to a predetermined extent and fixed in the resulting position and a clamping pressure should subsequently be applied to the other jaw so that the yielding movement to be expected will result in a fixation of the workpiece in the desired position. But such a so-called overstroke can be used merely to effect an approximated adaptation of the fixing apparatus but cannot be used for an exact fixation of the workpiece in the desired position. Besides, considerable non-productive times will be required for the movement of the jaws into engagement with the workpieces, particularly if the workpieces have different dimensions. the largest dimension of the workpiece must be taken into account in the design of means for slowing down the actuators from the rapid motion to the slow motion.